Exhaust diffuser and method for manufacturing an exhaust diffuser

ABSTRACT

An exhaust diffuser and method for manufacturing an exhaust diffuser is provided. An exhaust diffuser for a torque-generating turbine, in particular a torque-generating gas turbine is provided herein. The exhaust diffuser having an inner member, the inner member having an outer surface and an outer member having an inner surface, the inner member and the outer member forming an annular channel, at least a first supporting strut connecting the inner member and the outer member, the first supporting strut extending essentially radially from the inner surface to the outer surface, the first supporting strut having a middle section having a first airfoil and an outer section having a second airfoil, and the second airfoil differing from the first airfoil in shape to be able to handle a higher range of angle of attack. Furthermore, it is described a method for manufacturing an exhaust diffuser.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the US National Stage of International ApplicationNo. PCT/EP2012/065054 filed Aug. 1, 2012, and claims the benefitthereof. The International Application claims the benefit of EuropeanApplication No. 11178144 filed Aug. 19, 2011. All of the applicationsare incorporated by reference herein in their entirety.

FIELD OF INVENTION

The present invention relates to the field of torque-generatingturbines, in particular torque-generating gas turbines. Moreparticularly, the invention relates to an exhaust diffuser for atorque-generating turbine, in particular a torque-generating gasturbine, and a method for manufacturing an exhaust diffuser.

ART BACKGROUND

Torque generating turbines are rotary engines that extract energy from afluid flow, e.g. steam flow and/or combustion gas flow, and convert itinto useful work. Typically, torque-generating turbines are used todrive a generator to produce electric energy. Usually, atorque-generating turbine comprises a stator and a rotor having one orseveral turbine stages.

In a turbine stage fluid, e.g. steam or exhaust gas, is directed ontothe rotor by the fixed vanes of the stator. It leaves the stator as ajet that fills the entire circumference of the rotor. The flow thenchanges direction and increases its speed relative to the speed of theblades. A pressure drop occurs across both the vane and the blade, withflow accelerating through the vane and decelerating through the blade,with no significant change in flow velocity before and after the stage(vane inlet and blade outlet) but with a decrease in both pressure andtemperature, reflecting the work performed in the driving of the rotor.

Torque-generating gas turbines additionally comprise a compressor and acombustion chamber. The compressor having a stator and a rotor like theturbine typically has several stages as described in the paragraphabove. However, in the compressor both pressure and temperatureincreases from the first to the last stage, reflecting the workperformed by the turbine in the driving of the rotor of the compressor.The compressor compresses ambient air before it is mixed with fuel inthe combustion chamber to generate a burning mixture. The exhaust gas isthen fed into the turbine in the narrower sense where it expands. Aftera portion of the turbine, e.g. after 2 stages, an intermediate diffusermay be provided for reducing the speed of the fluid flow such that itsflow adapts smoothly to the geometry of the downstream turbine stages.After the last turbine stage there is a diffuser that decelerates theflow as much as possible to minimize the pressure drop and therebyimprove the performance of the turbine and the gas turbine as a wholebefore it is exhausted through the stack. The compressor also has adiffuser after its last stage to minimize the pressure drop. Therotating rotor of the turbine then drives the compressor and for examplea generator.

The junction of the vanes of the stators of either the compressor stagesor the turbine stages with the outer or inner surfaces may be sources ofhorseshoe vortexes and may thus impede, that the fluid flow is optimallyredirected onto the rotor.

EP 1 674 664 A2 relates to an exit guide vane array for thrustgenerating military aircraft engines, which includes a set of guidevanes having a solidity and defining fluid flow passages with achordwisely converging forward portion. The high solidity and convergentpassage portion resist fluid separation. The vanes cooperate with eachother to restrict an observer's line of sight of planes upstream of thevane array.

In EP 0798447 B1 a gas turbine guide vane with an enlarged filled radiusnear the end wall has been proposed to reduce the amount of draggenerating vortexes. Furthermore, WO 00/61918 suggests enlarging theleading edge radius near the end walls. In WO 2010/063271 it hasadditionally been proposed to enlarge the cross sectional area of strutssupporting annular channels connecting subsequent turbine stages of athrust generating aircraft gas turbine.

The fluid flow velocity after the rotor may still be significant.Accordingly, turbines are provided after the last rotor with an exhaustdiffuser to slow down the fluid flow and thereby enhance pressurerecovery.

The exhaust diffuser typically comprises an inner member having an outersurface and an outer member having an inner surface, wherein the innermember and the outer member form an annular channel. The inner memberhas to be supported within the outer member. Accordingly, the exhaustdiffuser comprises struts extending essentially radially from the outersurface of the inner member to the inner surface of the outer member.Typically, the struts have a prismatic design with a cross section thatdoes not change across the span from the outer surface of the innermember to the inner surface of the outer member.

The cross section of the strut typically has the form of an airfoil,i.e. it is shaped aerodynamically to reduce the drag induced by thestrut as far as possible. The strut often has more than one function.Maintaining the distance between the inner member and outer member,transfer forces from the rotor via the bearing to the casing, provideinspection access to the area inside the inner member, provide passagefor instrumentation wiring, supply and drain routes of air andlubrication oil to bearing.

Several parameters may characterize an airfoil: —chord line, —chord,—thickness, —mean camber line, —camber, —leading edge radius

The chord line is shortest line connecting leading edge and trailingedge of an airfoil. Accordingly, the chord denotes its length. Unlessotherwise stated, further dimensions of an airfoil are always givenrelative to the chord. The thickness of an airfoil indicates the maximumextension perpendicular to the chord line. The mean camber line is theline connecting the points midway between the upper surface and thelower surface of an airfoil along the chord line. The camber expressesthe maximum distance between the camber line and the chord line. Forsymmetric airfoils chord line and camber line are identical and thus thecamber is zero, of course. The leading edge radius is the radius, whichmay be fitted to the leading edge of the airfoil.

A fluid flow component at the inlet of the exhaust diffuser may comprisea component transversal to the exhaust diffuser, i.e. may enclose a flowangle different from zero with an axis of the annular channel of theexhaust diffuser. The flow angle may depend on the capacity of thetorque-generating turbine or its operating point, e.g. load or speed,and the radial distance of the fluid flow component from the axis of theannular channel of the exhaust diffuser.

The flow angle significantly affects pressure recovery of an exhaustdiffuser. It has been shown that an exhaust diffuser may show goodperformance up to a flow angle of 15 degree whereas steep losses occurthereafter.

The angle of incidence of an airfoil of conventional prismatic struts,i.e. the angle between the chord line and the axis of the exhaustdiffuser, is typically selected to be zero or to be equal to the meaninlet flow angle. Hence, the angle of attack, i.e. the angle between thefluid flow and the chord line of the airfoil may therefore vary acrossthe span of the prismatic strut. If the angle of attack becomes too highor too low flow separation may occur and large regions of low momentumfluid may be generated. These may lead to blockages and endangerpressure recovery of the exhaust diffuser. Accordingly, airfoilssupporting a large range of angle of attack, in particular airfoilshaving a large leading edge radius have been used hereinbefore. However,airfoils supporting a large range of angle of attack may have a higherdrag coefficient when the angle of attack is low thus rendering theexhaust diffuser less efficient.

US 2011/0052373 A1 proposes a supporting strut comprising a channel fromthe pressure side to the suction side of an airfoil to avoid flowseparation. However, this supporting strut has a significantcross-section when the angle of attack is low going along with a highdrag.

US 2009/0324400 A1 suggests a gas turbine for use in subsonic flightwherein a supporting strut for the thrust generating nozzle of the gasturbine is provided with a channel to reduce strut wake loss.

There may be a need for a more efficient exhaust diffuser being at thesame time less prone to blockages.

SUMMARY OF THE INVENTION

This need may be met by the subject matter described herein.Advantageous embodiments of the present invention are further describedherein.

According to a first aspect of the invention there is provided anexhaust diffuser for a torque-generating turbine, in particular atorque-generating gas turbine, the exhaust diffuser comprising an innermember having an outer surface and an outer member having an innersurface, the inner member and the outer member forming an annularchannel, at least a first supporting strut connecting the inner memberand the outer member, the first supporting strut extending essentiallyradially from the inner surface to the outer surface, the firstsupporting strut comprising a middle section having a first airfoil andan outer section having a second airfoil, and the second airfoildiffering from the first airfoil in shape to be able to handle a higherrange of angle of attack with respect to the first airfoil.

The range of angle of attack an airfoil may be able to handle depends onits shape. Typical parameters, which may influence this property, areinter alia leading edge radius, trailing edge radius, camber. A personskilled in the art knows the range of angle of attack a particularairfoil may handle. Hence, a person skilled in the art may select anairfoil based on this parameter.

A second airfoil having a shape capable of handling a higher range ofangle of attack may help to avoid flow separation near the inner surfaceof the outer member, where the flow angle may change rapidly compared tothe flow angle in the middle between outer surface of the inner memberand inner surface of the outer member. Hence, diffuser blockages may beavoided and pressure recovery enhanced.

According to a first embodiment of the exhaust diffuser for atorque-generating turbine the first supporting strut comprises an innersection having a third airfoil and the third airfoil differs from thefirst airfoil in shape to be able to handle a higher range of angle ofattack with respect to the first airfoil.

The flow angle near the outer surface of the inner member may deviatefrom the flow angle in the middle between the outer surface of the innermember and the inner surface of the outer member, too. A third airfoilhaving a shape capable of handling a higher range of angle of attack mayreduce the amount of low momentum fluid in this region and furtherimprove pressure recovery.

According to a second embodiment of the exhaust diffuser for atorque-generating turbine the first airfoil has a smaller thickness thanthe second airfoil and/or the third airfoil.

A thinner first airfoil may have a lower drag coefficient than the firstairfoil and/or the third airfoil. Accordingly, pressure recovery alongthe exhaust diffuser may be improved. Hence, a torque-generatingturbine, in particular a torque-generating gas turbine, provided withsuch an exhaust diffuser may be more efficient.

According to another embodiment of the exhaust diffuser for atorque-generating turbine the first airfoil has a smaller leading edgeradius than the second airfoil and/or the third airfoil.

Although an airfoil with a smaller leading edge radius may support alower range of angle of attack it may have the advantage of lower dragand thus to improve the efficiency of the exhaust diffuser.

However, a smaller leading edge radius does not necessarily go alongwith a lower range of angle of attack and vice versa as other parametersof the airfoil may influence its aerodynamics, too.

According to yet another embodiment of the exhaust diffuser for atorque-generating turbine the first airfoil has a smaller camber thanthe second airfoil and/or the third airfoil.

While a higher camber may allow for supporting a higher range of angleof attack, a smaller camber may reduce the drag of the first airfoil.Flow angle deviations in the middle section of the supporting strut maybe less severe.

Accordingly, the permissible range of angle of attack may be lessimportant than the drag coefficient. In particular, the camber of thefirst airfoil may be zero, i.e. the first airfoil may be a symmetricairfoil. A symmetric airfoil may result in very low drag if the angle ofattack is approximately zero.

According to a further embodiment of the exhaust diffuser for atorque-generating turbine the second airfoil has a higher angle ofincidence than the first airfoil.

The flow angle may be higher near the inner surface of the outer member.Accordingly, a higher angle of incidence of the second airfoil mayresult in a lower angle of attack. A lower angle of attack may lead tolower drag and may thus enhance pressure recovery of the exhaustdiffuser.

According to a still further embodiment of the exhaust diffuser for atorque-generating turbine the third airfoil has a lower angle ofincidence than the first airfoil and/or the second airfoil.

Near the outer surface of the inner surface the flow angle duringoperation of the torque-generating turbine may be very low, inparticular negative. Adapting the angle of incidence of the thirdairfoil such that it is lower than the angle of incidence of the firstairfoil and/or the second airfoil may therefore result in a lower angleof attack at the inner section of the supporting strut reducing drag andaugmenting pressure recovery of the exhaust diffuser.

According to another embodiment of the exhaust diffuser for atorque-generating turbine the difference between the angle of incidenceof the third airfoil and the angle of incidence of the second airfoil isbetween 0° and 40°, in particular between 5° and 30°, more particularlybetween 5° and 20°.

A difference in angle of incidence between 0° and 40° may beadvantageous in case of a torque-generating turbine being operated in awide power range to respect the significant deviations in flow angleassociated therewith. A range of 5° to 30° may be preferred when thetorque-generating turbine is only operated in a limited power range andthus changing flow angles are less common. Selecting the difference inangle of incidence to be between 5° and 20° may simplify manufacturingof the supporting strut.

According to yet another embodiment of the exhaust diffuser for atorque-generating turbine the exhaust diffuser comprises a secondsupporting strut and the first supporting strut and the secondsupporting strut are arranged such as to form a twin-strut with an inflow-direction narrowing channel therein between.

A twin-strut may be regarded as a first supporting strut and a secondsupporting strut being arranged side-by-side in close proximity to oneanother.

A twin-strut comprising an in flow-direction narrowing channel mayreduce the risk of flow separation even under severe angle of attack.

According to a further embodiment of the exhaust diffuser for atorque-generating turbine the first supporting strut and the secondsupporting strut form a mirror symmetric twin-strut.

A mirror symmetric twin-strut may particularly easy to design and tomanufacture.

According to another embodiment of the exhaust diffuser for atorque-generating turbine the exhaust diffuser has an even number offirst supporting struts and/or second supporting struts.

An even number of first supporting struts and/or second supportingstruts may facilitate the arrangement of said struts around thecircumference of the inner member.

According to yet another embodiment of the exhaust diffuser for atorque-generating turbine the exhaust diffuser has an odd number offirst supporting struts and/or second supporting struts.

An odd number of first supporting struts and/or second supporting strutsmay result in a lower total number of said struts and thus in lesserdrag.

According to a still further embodiment of the exhaust diffuser for atorque-generating turbine the exhaust diffuser has eight or fewer firstsupporting struts and/or second supporting struts, in particular six orfewer first supporting struts and/or second supporting struts.

Reducing the number of supporting struts may reduce drag losses.Accordingly, the efficiency of the exhaust diffuser may be improved. Thesupporting struts may all have the same shape or differ in theirappearance.

According to another embodiment of the exhaust diffuser for atorque-generating turbine the exhaust diffuser has four or fewertwin-struts, in particular three or fewer twin-struts.

Twin-struts may further help to reduce the amount of supporting strutswhile maintaining the structural stability of the exhaust diffuser.

According to a second aspect of the invention there is provided a methodfor manufacturing an exhaust diffuser for a torque-generating turbine ashas been described hereinbefore.

According to this description “inner” means radial inwards in respect ofan axis of rotation of the turbine; “outer” means radial outwards inrespect of an axis of rotation of the turbine. Particularly in adiverging annulus, “inner member” means a part that defines a firstboundary of the annulus and “outer member” means a part that defines asecond boundary of the annulus, wherein the second boundary has agreater distance to an axis of rotational symmetry than the firstboundary.

“Higher” angle of incidence of a first airfoil in comparison to a secondairfoil may mean that compared to the direction of the mean inlet flowof the diffuser inlet of the gas turbine, the first airfoil has agreater angle between its chord line and the mean inlet flow than anangle between a chord line of the second airfoil and the mean inletflow.

Besides “higher angle” may mean that the absolute value of a first angleis greater than the absolute value of a second angle. Thus, thealgebraic sign of the angle is disregarded so that is does not matterwhether is section is twisted clockwise or counter-clockwise.

“Higher” angle of attack of a first airfoil in comparison to a secondairfoil may mean that compared to the direction of local fluid flow inthe diffuser of the gas turbine, the first airfoil has a different anglebetween its chord line and the direction of the local fluid flow passingalong the first airfoil than an angle between a chord line of the secondairfoil and the direction of the local fluid flow passing along thesecond airfoil.

The first and the second airfoil may be a common part but may onlydefine different sections of the common part.

The torque-generating turbine may particularly be a gas turbine engine.

It has to be noted that embodiments of the invention have been describedwith reference to different subject matter. In particular, someembodiments have been described with reference to a method whereas otherembodiments have been described with reference to an apparatus. However,a person skilled in the art will gather from the above and the followingdescription that, unless otherwise notified, in addition to anycombination of features belonging to one type of subject matter also anycombination between features relating to different subject matters, inparticular between features of the method and features of the apparatusare considered as to be disclosed herein.

The aspects defined above and further aspects of the present inventionare apparent from the examples of embodiment to be described hereinafterand are explained with reference to the examples of embodiment. Theinvention will be described in more detail hereinafter with reference toexamples of embodiment but to which the invention is not limited.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows the flow angle of a fluid flow entering an exhaust diffuserversus the normalized distance from the outer surface of its innermember to the inner surface of its outer member for three differentcapacities of a typical torque-generating turbine.

FIG. 2 shows a prismatic supporting strut for an exhaust diffuser.

FIG. 3 shows an exemplary embodiment of a supporting strut comprising aninner section, a middle section, and an outer section having differingairfoils.

FIG. 4 shows a pressure recovery coefficient along the axial extension Lof a first exhaust diffuser D₁ and a second exhaust diffuser D₂.

FIG. 5 shows a further prismatic strut.

FIG. 6 shows yet another exemplary embodiment of a supporting strut foran exhaust diffuser.

FIG. 7 shows a pressure recovery coefficient along the axial extension Lof a third exhaust diffuser D₃ and a forth exhaust diffuser D₄.

FIG. 8 shows a visualization of streamlines of an exhaust diffusercomprising single angle of incidence supporting struts at 10 percent oftheir span.

FIG. 9 shows a visualization of streamlines of an exhaust diffusercomprising twisted supporting struts at 10 percent of their span.

FIG. 10 shows in a very schematic form a first supporting strut and asecond supporting strut forming a twin-strut at an angle of attack closeto zero.

FIG. 11 shows in a very schematic form a first supporting strut and asecond supporting strut forming a twin-strut at an angle of attacksignificantly different from zero.

DETAILED DESCRIPTION

The illustration in the drawing is schematically provided.

FIG. 1 shows the flow angle a of the fluid flow entering an exhaustdiffuser versus the normalized distance D from the outer surface of itsinner member to the inner surface of its outer member for a low, middleand high capacity of a torque-generating gas turbine. For a middlecapacity the flow angle varies from −27 degree at the outer surface ofthe inner member of the exhaust diffuser to approximately 10 degree atthe inner surface of the outer member of the exhaust diffuser. The meanflow angle is approximately 5 degree, which allows for good diffuserrecovery.

In particular, near the outer surface and the inner surface the flowangle gradient is quite extreme. Accordingly, it may be advantageous totreat these end wall regions differently.

FIG. 2 shows a prismatic supporting strut 1 according to the state ofthe art comprising an inner section 2, a middle section 3, and an outersection 4 all having the same airfoil.

FIG. 3 shows a first exemplary embodiment of a supporting strut 5 for anexhaust diffuser according to the invention. The supporting strutcomprises a middle section 7 having a first airfoil, an outer section 6having a second airfoil and an inner section 8 having a third airfoil.

The first airfoil has a lower thickness than the second airfoil and thethird airfoil. The lower thickness of the first airfoil leads to a lowerdrag coefficient when the angle of attack is low in particular in themiddle section 7 of the strut. The second airfoil and the third airfoilhave a larger leading edge radius. Accordingly, the airfoil may resemblea dumbbell when viewed in the axial direction. The second airfoil andthe third airfoil may handle a higher range of angle of attack. Hence,the despite the large deviations of flow angle near the outer surface ofthe inner member and the inner surface of the outer member flowseparation may be avoided.

Only the circumference of the supporting strut 5 is shown in FIG. 3. Thesupporting strut 5 may be solid or hollow. A hollow design may beadvantageous to guide tube, cables or fibers therethrough. Theseelements may provide the inner member with lubrication, energy and/orcontrol data. Furthermore, energy and sensor data may be received fromthe inner member.

Avoiding flow separation may reduce exhaust diffuser blockage and mayenhance pressure recovery as illustrated in FIG. 4. This diagram showsthe pressure recovery coefficient CP along the axial extension L of afirst exhaust diffuser D₁ and a second exhaust diffuser D₂. The firstexhaust diffuser D₁ comprises supporting struts with a prismatic designas shown in FIG. 2 in a distance L₁ to L₂ from the beginning of theexhaust diffuser. The second exhaust diffuser D₂ comprises the samenumber of supporting struts at the same distance from the beginning ofthe exhaust diffuser but with a “dumbbell” design as shown in FIG. 3. Inboth cases the pressure recovery coefficient drops in the region of thesupporting struts. However, pressure recovery coefficient drop is lesssevere in the case of the “dumbbell” design. Thus, even after thesupport strut a higher pressure recovery coefficient may be achievedwith the “dumbbell” design.

FIG. 5 shows a single incidence supporting strut 9. A line A indicatesthe axis of the exhaust diffuser.

FIG. 6 shows another exemplary embodiment of a supporting strut 10 foran exhaust diffuser comprising a middle section 11 having a firstairfoil, an outer section 12 having a second airfoil and an innersection 13 having a third airfoil. A line B indicates the axis of theexhaust diffuser.

The first airfoil has a lower thickness than the second airfoil and thethird airfoil. The lower thickness of the first airfoil leads to a lowerdrag coefficient when the angle of attack is low in particular in themiddle section 11 of the strut. The second airfoil and the third airfoilhave a larger leading edge radius. The second airfoil and the thirdairfoil may handle a higher range of angle of attack. Hence, despite thelarge deviations of flow angle near the outer surface of the innermember and the inner surface of the outer member flow separation may beavoided.

Furthermore, the second airfoil and the third airfoil have anorientation different from the first airfoil. The second airfoil isadapted to a higher flow angle than the first airfoil and the thirdairfoil is adapted to a lower, in particular negative, flow angle thanthe first airfoil.

The twisted supporting strut with changing incidence of first airfoil,second airfoil and third airfoil like that shown in FIG. 6 may improvethe pressure recovery coefficient compared to a standard single angle ofincidence supporting strut 9 as shown in FIG. 5.

FIG. 7 shows the pressure recovery coefficient CP along the axialextension L of a third exhaust diffuser D₃ and a forth exhaust diffuserD₄. Both third exhaust diffuser D₃ and forth exhaust diffuser D₄comprise supporting struts extending in a distance L₃ to L₄ from thebeginning of the respective exhaust diffuser. The supporting struts ofthe third exhaust diffuser D₃ are single angle of incidence supportingstruts. On the other hand the supporting struts of the forth exhaustdiffuser D₄ twisted supporting struts like the one shown in FIG. 6. Bothexhaust diffusers D₃ and D₄ experience a pressure recovery coefficientdrop at the beginning of the supporting struts. However, the pressurerecovery coefficient rises more quickly again in case of the exhaustdiffuser D₄ comprising the twisted struts.

FIG. 8 shows a visualization of streamlines at 10 percent span of asingle angle of incidence supporting strut 14 of an exhaust diffuser.Flow separation occurs, leaving behind horseshoe vortexes 15 indicativeof flow separation. This results in regions of low momentum fluid andpartly exhaust diffuser blockage.

FIG. 9 shows a visualization of streamlines under the same conditionsbut with twisted supporting struts 16. No horseshoe vortexes arevisible. Hence, pressure recovery may be enhanced.

FIGS. 10 and 11 show in a very schematic form a first supporting strut17, 20 and a second supporting strut 18, 21 wherein the first supportingstrut 17, 20 and the second supporting strut 18, 21 are arranged such asto form a twin-strut with an in flow-direction narrowing channel 19, 22therein between.

The first supporting strut 17, 20 and the second supporting strut 18, 21are strategically placed side-by-side in a mirrored configurationcomplementing each other. The distance therein between is optimized forthe flow rate, and the distribution of the area from the leading edge23, 24 to the trailing edge 25, 26 of the so-formed twin-strut isoptimized such that the flow exiting the channel 19, 22 experiencesacceleration to match the flow at the outside of the channel 19, 22. Thefirst supporting strut 17, 20 and the second supporting strut 18, 21will match each other to form an aerodynamically efficient profile.

At design flow angle the flow will treat the twin-strut as twoindividual supporting 17, 18 struts along the outer surfaces of thetwin-strut. The channel 19 will keep the flow accelerated sufficientlytowards the exit for low trailing edges losses. The slim leading edge23, 24 and slim trailing edge 25, 26 of each of the supporting struts17, 18 may reduce profile loss, i.e. loss due to the shape of theairfoil.

When the flow angle deviates from the design flow angle, i.e. the angleof attack deviates significantly from zero, the flow will primarilyreach the outer surface of one of the supporting struts 20 of the pair.The twin-strut will look for the flow as a single supporting strut witha large leading edge radius. This large leading edge radius will reducethe risk of flow separation even at large angle of attack while any flowin the channel will again smoothly guide flow back to the exhaustdiffuser axis at the trailing edge to reduce mixing losses.

The comparable large area presented by the twin-strut design may allowfor lowering the number of struts connecting the inner member and theouter member without compromising the mechanical stability of theexhaust diffuser. Instead of having 6 individual struts, 3 twin-strutsmay be sufficient.

Pressure recovery may still further be improved by increasing the exitradius of the exhaust diffuser relative to the inlet radius of theexhaust diffuser.

It should be noted that the term “comprising” does not exclude otherelements or steps and the use of articles “a” or “an” does not exclude aplurality. Also elements described in association with differentembodiments may be combined. It should also be noted that referencesigns in the claims should not be construed as limiting the scope of theclaims.

The invention claimed is:
 1. An exhaust diffuser for a torque-generatingturbine, the exhaust diffuser comprising: an inner member having anouter surface and an outer member having an inner surface, the innermember and the outer member forming an annular channel, a firstsupporting strut connecting the inner member and the outer member, thefirst supporting strut extending essentially radially from the innersurface to the outer surface, a second supporting strut, wherein thefirst supporting strut and the second supporting strut form a mirrortwin-strut with an in flow-direction narrowing channel therein between;the first supporting strut and the second supporting strut comprising amiddle section having a first airfoil, an outer section having a secondairfoil and an inner section having a third airfoil, the second airfoildiffering from the first airfoil in shape to be able to handle a higherrange of angle of attack with respect to the first airfoil, and wherein:the first airfoil comprises a smaller leading edge radius than thesecond airfoil and the third airfoil; or the first airfoil comprises asmaller camber than the second airfoil and the third airfoil; or thesecond airfoil comprises a higher angle of incidence than the firstairfoil and the third airfoil comprises a lower angle of incidence thanthe first airfoil.
 2. The exhaust diffuser for a torque-generatingturbine of claim 1, the third airfoil differing from the first airfoilin shape to be able to handle a higher range of angle of attack withrespect to the first airfoil.
 3. The exhaust diffuser for atorque-generating turbine of claim 2, the first airfoil having a smallerthickness than the second airfoil and/or the third airfoil.
 4. Theexhaust diffuser for a torque-generating turbine of claim 2, the firstairfoil having the smaller leading edge radius than the second airfoiland/or the third airfoil.
 5. The exhaust diffuser for atorque-generating turbine of claim 2, the first airfoil having thesmaller camber than the second airfoil and/or the third airfoil.
 6. Theexhaust diffuser for a torque-generating turbine of claim 1, the secondairfoil having the higher angle of incidence than the first airfoil. 7.The exhaust diffuser for a torque-generating turbine of claim 2, thethird airfoil having the lower angle of incidence than the firstairfoil.
 8. The exhaust diffuser for a torque-generating turbine ofclaim 1, wherein the exhaust diffuser has an even number of firstsupporting struts and/or second supporting struts.
 9. The exhaustdiffuser for a torque-generating turbine of claim 1, wherein the exhaustdiffuser has an odd number of first supporting struts and/or secondsupporting struts.
 10. The exhaust diffuser for a torque-generatingturbine of claim 1, comprising eight or fewer first supporting strutsand/or second supporting struts.
 11. The exhaust diffuser for atorque-generating turbine of claim 1, wherein the exhaust diffuser hasfour or fewer twin-struts.
 12. The exhaust diffuser for atorque-generating turbine of claim 1, comprising six or fewer firstsupporting struts and/or second supporting struts.
 13. The exhaustdiffuser for a torque-generating turbine of claim 1, wherein the exhaustdiffuser has three or fewer twin-struts.
 14. The exhaust diffuser for atorque-generating turbine of claim 1, wherein the torque-generatingturbine is a torque-generating gas turbine.
 15. A torque-generatingturbine comprising, in order of a gas flow, a compressor, a combustor, aturbine and the exhaust diffuser as claimed in claim
 1. 16. Atorque-generating turbine as claimed in claim 15 and having a stack, thestack located downstream of the exhaust diffuser.
 17. Atorque-generating turbine as claimed in claim 15, wherein the turbinehas at least two stages, one stage driving the compressor and anotherstage driving a generator.